Silicon nitride ceramics are being widely considered as materials for components in advanced energy conversion systems due to their high temperature properties, including resistance to corrosion and erosion. The use of silicon nitride materials permits higher operating temperatures in such processes, permitting increased engine efficiency, more economic use of high grade fuels and the possibility of burning lower grade fuels derived from residual oils and coal. The gain in efficiency resulting from higher gas inlet temperatures has led to the emergence of the small ceramic gas turbine as a serious competitor to the internal combustion engine for many automotive applications. Silicon nitride materials, for the same reasons, are being considered for use in advanced Sterling engines and high temperature diesel engines.
However, despite the attractive properties of silicon nitride in such high technology applications, certain problems still exist in the manufacture and fabrication of silicon nitride components which inhibit its widespread use.
In many instances it is impractical to fabricate components in one piece. In such cases it is necessary to join parts together either mechanically or by some chemical bonding technique. The mechanical joining obviously is impractical or impossible in many high temperature applications.
Bonding techniques to date have not been as successful as desired. Ideally any technique for bonding or sealing silicon nitride to itself should produce a bond at least as strong and as chemically resistant as the materials joined. This has proved difficult to achieve in practice. Methods have been used which include brazing, reactive solid state sintering, and sealing with reactive liquids. For example, metal alloys with the proper thermal expansion coefficients and the ability to wet silicon nitride have been used to braze silicon nitride to itself or to other materials. However, in most cases the metal bond is neither as refractory nor as resistant to oxidation and chemical attack as the silicon nitride.
In the case of reaction bonded silicon nitride, joining can be accomplished by nitriding a porous layer of silicon placed between the silicon nitride pieces to be joined together. However, this technique is limited to relatively small cross sectional areas at the bond and cannot be used for other types of silicon nitride such as hot pressed or sintered material.
It is also possible to satisfactorily join silicon nitride parts together by hot pressing the parts in direct contact or with a layer of suitable bonding material between them. While such a bond is possible in principle, it is extremely impractical in practice due to the complexity and expense of the procedure, including the need for proper alignment of the parts to be joined. Also the components frequently fracture or deform while being joined.
Other sealing techniques not requiring the use of such high pressures have also been developed. Typically such processes involve the use of other refractory materials such as alkaline earth-aluminum silicate glasses. However, these materials may not provide reliable bonds because of potential mismatch of the thermal expansion coefficients of the glass and the materials sealed, and much lower mechanical strength in the glass bond compared with metal or ceramic bonds.
It has now been discovered that a homogeneous bond can be formed between silicon nitride materials without encountering the problems discussed.